diff --git a/nanobot/channels/manager.py b/nanobot/channels/manager.py
index 4b16774a8..f5ad7a04a 100644
--- a/nanobot/channels/manager.py
+++ b/nanobot/channels/manager.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 import asyncio
 import hashlib
 import inspect
+from collections import OrderedDict
 from collections.abc import Awaitable, Callable, Iterable, Mapping
 from contextlib import suppress
 from pathlib import Path
@@ -60,6 +61,7 @@ def _default_webui_dist() -> Path | None:
 _SEND_RETRY_DELAYS = (1, 2, 4)
 _RESTART_NOTICE_START_TIMEOUT_S = 30.0
 _RESTART_NOTICE_START_POLL_S = 0.25
+ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE = 1000
 
 _BOOL_CAMEL_ALIASES: dict[str, str] = {
     "send_progress": "sendProgress",
@@ -137,7 +139,7 @@ class ChannelManager:
         self._channel_tasks: dict[str, asyncio.Task[None]] = {}
         self._dispatch_task: asyncio.Task[None] | None = None
         self._started = False
-        self._origin_reply_fingerprints: dict[tuple[str, str, str], str] = {}
+        self._origin_reply_fingerprints: OrderedDict[tuple[str, str, str], str] = OrderedDict()
 
         self._init_channels()
 
@@ -669,6 +671,16 @@ class ChannelManager:
         normalized = " ".join(content.split())
         return hashlib.sha1(normalized.encode("utf-8")).hexdigest() if normalized else ""
 
+    def _remember_origin_reply_fingerprint(
+        self,
+        key: tuple[str, str, str],
+        fingerprint: str,
+    ) -> None:
+        self._origin_reply_fingerprints[key] = fingerprint
+        self._origin_reply_fingerprints.move_to_end(key)
+        while len(self._origin_reply_fingerprints) > ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE:
+            self._origin_reply_fingerprints.popitem(last=False)
+
     def _should_suppress_outbound(self, msg: OutboundMessage) -> bool:
         metadata = msg.metadata or {}
         if isinstance(outbound_event_from_message(msg), ProgressEvent):
@@ -681,13 +693,14 @@ class ChannelManager:
         if isinstance(origin_message_id, str) and origin_message_id:
             key = (msg.channel, msg.chat_id, origin_message_id)
             if self._origin_reply_fingerprints.get(key) == fingerprint:
+                self._origin_reply_fingerprints.move_to_end(key)
                 return True
-            self._origin_reply_fingerprints[key] = fingerprint
+            self._remember_origin_reply_fingerprint(key, fingerprint)
 
         message_id = metadata.get("message_id")
         if isinstance(message_id, str) and message_id:
             key = (msg.channel, msg.chat_id, message_id)
-            self._origin_reply_fingerprints[key] = fingerprint
+            self._remember_origin_reply_fingerprint(key, fingerprint)
 
         return False
 
diff --git a/tests/channels/test_channel_plugins.py b/tests/channels/test_channel_plugins.py
index a223c37bb..6e5ac47c9 100644
--- a/tests/channels/test_channel_plugins.py
+++ b/tests/channels/test_channel_plugins.py
@@ -7,6 +7,7 @@ import json
 import subprocess
 import sys
 import tomllib
+from collections import OrderedDict
 from dataclasses import replace
 from importlib.metadata import PackageNotFoundError
 from pathlib import Path
@@ -32,7 +33,7 @@ from nanobot.channels.contracts import (
     SetupRequirement,
     channel_default_config,
 )
-from nanobot.channels.manager import ChannelManager
+from nanobot.channels.manager import ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE, ChannelManager
 from nanobot.channels.plugin import ChannelPlugin, load_channel_package
 from nanobot.config.loader import load_config, save_config
 from nanobot.config.schema import ChannelsConfig, Config
@@ -3179,7 +3180,7 @@ def test_outbound_duplicate_suppression_is_scoped_to_origin_message() -> None:
     mgr.bus = MessageBus()
     mgr.channels = {}
     mgr._dispatch_task = None
-    mgr._origin_reply_fingerprints = {}
+    mgr._origin_reply_fingerprints = OrderedDict()
 
     first = OutboundMessage(
         channel="feishu",
@@ -3212,6 +3213,39 @@ def test_outbound_duplicate_suppression_is_scoped_to_origin_message() -> None:
     assert mgr._should_suppress_outbound(new_origin_content) is False
 
 
+def test_outbound_duplicate_suppression_cache_is_bounded() -> None:
+    mgr = ChannelManager.__new__(ChannelManager)
+    mgr._origin_reply_fingerprints = OrderedDict()
+
+    for index in range(ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE):
+        msg = OutboundMessage(
+            channel="feishu",
+            chat_id="chat123",
+            content="Done",
+            metadata={"message_id": f"msg-{index}"},
+        )
+        assert mgr._should_suppress_outbound(msg) is False
+
+    duplicate = OutboundMessage(
+        channel="feishu",
+        chat_id="chat123",
+        content="Done",
+        metadata={"origin_message_id": "msg-0"},
+    )
+    newest = OutboundMessage(
+        channel="feishu",
+        chat_id="chat123",
+        content="Done",
+        metadata={"message_id": f"msg-{ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE}"},
+    )
+
+    assert mgr._should_suppress_outbound(duplicate) is True
+    assert mgr._should_suppress_outbound(newest) is False
+    assert len(mgr._origin_reply_fingerprints) == ORIGIN_REPLY_FINGERPRINTS_MAX_SIZE
+    assert ("feishu", "chat123", "msg-0") in mgr._origin_reply_fingerprints
+    assert ("feishu", "chat123", "msg-1") not in mgr._origin_reply_fingerprints
+
+
 @pytest.mark.asyncio
 async def test_send_with_retry_propagates_cancelled_error():
     """_send_with_retry should re-raise CancelledError for graceful shutdown."""